Certain materials such as various polymeric materials are capable of being polarized when subjected to mechanical or electrical stresses. In the past, it has been found that a polymeric material such as poly(vinylidene fluoride) can be polarized by stretching a sheet at a temperature of about 70.degree. C. at least three times its length, and subjecting the stretched sheet to a DC field of at least 1 MV/cm. Poly(vinylidene fluoride) has been a preferred material for polarization, since it has been found to have a high capability of polarization response, thereby providing high piezoelectric or pyroelectric properties or highly desired optical properties. Subjecting such a stretched film, for example, using an appropriate DC field applied in a direction perpendicular to the plane of the stretched film causes an orientation of the molecular dipoles of the materials. In the case of poly(vinylidene fluoride), the fluoro groups have a negative charge and the hydrogen atoms attached to the other carbon of the vinylidene fluoride unit of the polymer have a positive charge. Vinylidene fluoride units in a poly(vinylidene fluoride) can exist in at least two different crystalline forms. In one form, the vinylidene fluoride units exist in a planar zigzag polar form or trans form (beta form or Form 1). In another form, the form is nonpolar and nonplanar; it is a T-G-T-G' form (alpha form or Form 2) wherein T denotes trans configuration and G and G' denote the two types of gauche forms. In the past, the desired increase in Form 1 has been realized by subjecting poly(vinylidene fluoride) films (PVF.sub.2 films) to stretching and subsequently subjecting the stretched films to high DC fields over extended periods of time at high temperatures. Such treatment with a DC field is referred to as "poling." It is desired to have a high content of Form 1 in order to have the highest amount of desired polarization properties, for example, piezoelectric and pyroelectric properties. The polarized material is cooled after poling for purposes of retaining the polarization.
Such polarized materials are used, for example, in making transducers, which utilize the piezoelectric or pyroelectric or other polarization properties of such polarized materials.
Various other polymers, such as polyvinylchloride (PVC), polyvinylfluoride (PVF), vinylidene fluoride copolymers, and many other polymer materials have the capability of being polarized as do various non-polymeric materials such as some ceramic materials.
Customary stretching in the film direction causes an unequal (or anisotropic) strength in the stretching or axial direction (X-X.sup.1) as compared to the transverse direction (y-y.sup.1). This is undesirable. It is desired to provide materials which are free or substantially free of such mechanically induced orientation and which have a polarization which is stable up to the crystal melting point or material softening point, in the case of non-crystalline polarized materials. Such materials are substantially free of said anisotropic mechanical properties. Such polarized materials and processes for producing such polarized materials are highly desired.